A thermoelectric device can be used to obtain electrical energy from a thermal gradient (for example, a thermoelectric generator using the Seebeck effect), or to generate a thermal gradient from electrical energy (for example, a thermoelectric refrigerator using the Peltier effect). The discussion below is directed to the Seebeck effect, but the general concepts also apply to applications of the Peltier effect.
A typical thermoelectric device is built up from several unicouples, which are typically pairs of thermally conductive p-type (P) and n-type (N) semiconductors. These unicouples are connected electrically in series and thermally in parallel. Theoretically, the maximum efficiency of the conversion of heat energy to electrical energy is given by:
      ζ    max    =                    (                              T            H                    -                      T            C                          )                    T        H              ⁢                                        1            +                          ZT              ave                                      -        1                                          1            +                          ZT              ave                                      +                              T            C                    /                      T            H                              
where Tave=(TH+TC)/2 is the average temperature, and Z is a figure of merit, defined as Z=S2σ/κ. The figure of merit Z depends on the macroscopic transport parameters of the materials, namely the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ). A large figure of merit is provided by a thermoelectric material having a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity.
The Seebeck coefficient is further defined as the ratio of the open-circuit voltage to the temperature difference between the hot and cold junctions of a circuit exhibiting the Seebeck effect, or S=V/(TH−TC). Since Z varies with temperature, a useful dimensionless figure-of-merit can be defined as ZT.
By the end of the 1950s, the best bulk thermoelectric materials were found to be alloys of bismuth telluride and antimony, which gave a room temperature ZT˜1. Workers in the thermoelectric field have been attempting to improve the figure of merit over the past 40 years without much success. Increasing ZT is difficult because the three parameters S, σ, and k are all related to the free carrier concentration and are usually not independent. For example, doping typically increases the semiconductor's electrical conductivity, but decreases its Seebeck coefficient and increases the thermal conductivity. Efforts to reduce the lattice thermal conductivity by alloying also reduce the electrical conductivity by providing an extra scattering mechanism.
Dresselhaus and coworkers at MIT theoretically demonstrated that quantum confinement of electrons and phonons within nanowires of a thermoelectric material can increase the value of ZT. 1-D nanowires in particular could reach ZT≈2-5 if the nanowire diameter lies in the range of 5-10 nm. Certain structures have been investigated, for example such as described in Heremans, J. P. et al., “Thermoelectric Power of Bismuth Nanocomposites”; Phys. Rev. Lett.; 2002, 88, 216801; Venkatasubramanian, R. et al., “Thin-film thermoelectric devices with high room temperature figures of merit”; Nature; 2001, 413, 597-602; Harman, T. C. et al., “Thermoelectric quantum dot superlattices with high ZT”; Electron. Mater.; 2000, 29, L1-L4; Rabin, O. et al., “Anomalously high thermoelectric figure of merit in Bi1-xSbx nanowires by carrier pocket alignment”; APL; 2001, 79, 81-83; and Dresselhaus, M. S. et al., “Low-dimensional thermoelectric materials”; PSS; 1999, 41, 679-682. However, these approaches do not provide a simple approach to making large-scale, low-cost thermoelectric devices. Conventional semiconductor device fabrication methods are unsuitable for manufacturing bulk samples, and are often expensive.
In automobiles, about 70 percent of energy derived from fuel is lost to waste heat and engine cooling. Only a small proportion of energy provided by fuel combustion is used, and a large amount of thermal energy is thrown away. Recovery of waste thermal energy is a big challenge in automotive industries due to the increasing energy crisis. Thermoelectric conversion of thermal energy to electrical energy could be an effective way to obtain electrical energy from otherwise wasted heat production. However, direct thermal to electric conversion (DTEC) technology currently faces two major challenges: low conversion efficiency and insufficient power density. Hence, improved materials and devices having high thermoelectric conversion efficiency are urgently required.